When two genes are perturbed simultaneously, a surprising phenotype often emerges. Genetic interaction-defined by this phenomenon-suggests that the interacting genes have related functions. Genetic interactions have shaped our understanding of nearly all known biological pathways. Examples of genetic interaction encompass complex human diseases such as cancer that require multiple mutations. The multigenic origins of such diseases motivate the need to discover and understand how genetic interactions yield extreme phenotypes. Due to its facile genetics, S. cerevisiae has been a key model organism for the systematic study of genetic interactions. Technology that can more efficiently map genetic interactions would accelerate completion of the current interaction mapping effort to encompass all -18 million gene pairs in yeast. Moreover, genetic interactions are strongly influenced by or dependent on environment. Recent advances in next-generation DNA sequencing technology have reduced the cost of sequencing short sequence tags by at least a factor of 1000 relative to conventional Sanger dideoxy sequencing coupled with capillary electrophoresis. Future improvements in scale and cost are imminently expected over the next several years. Here I propose to develop and apply a new approach termed 'barcode fusion genetics'BFG) on a pilot scale to study factors influencing DNA repair in yeast. This project would demonstrate :feasibility for a technology that could produce a global genetic interaction map in a particular environmental condition by a single technician within a year. Effects on DNA maintenance and cellular response by endogenous and exogenous mutagenesis is both of basic science interest and is also critical to our understanding of cancer etiology in humans. There are well over a hundred genes in yeast that encode different parts of the DNA repair process, including protein complexes for recognizing different forms of DNA breaks and lesions, signaling proteins for activating specific repair pathways, and enzymes for catalyzing repair operations. Applied to the unraveling of genetic interactions among DNA repair genes, the proposal outlined here will significantly enhance our understanding of this crucial biological function.